To help students make reasonable estimates with any type of measurement including weight, students must have experiences measuring items that are common to them. For example, when teaching students how heavy a pound or ounce is, allow them to hold a small bag of sugar for a pound and a square of baking chocolate for an ounce. Have students predict how many baking chocolate squares (ounces) it will take to make a bag of sugar (pound). Then place one ounce on the scale at a time for students to see. When you are halfway to a pound stop and ask the students if they wish to change their predictions.
Notes: When students only hold the standard metal measurements that come with balance beam scales, they tend to have a skewed view of how heavy a pound or ounce is since the metal is so dense. When you are looking for a pound of sugar in the baking aisle, the normal granulated sugar isn’t typically sold in a one pound bag. However, you can buy a small pound of sugar in a box to represent one pound. These are the only items in the grocery store that I have found to weigh exactly one pound and one ounce. If you have found others, please comment.